This study examines the drug interaction between 566C80, an investigational drug for the treatment of Pneumocystis carinii pneumonia, and dideoxyinosine (ddI), an approved drug for the treatment of HIV infection. 566C80 has been shown to be effective in the treatment of mild to moderate Pneumocystis carinii pneumonia. ddI is commonly given to patients with HIV infection who are also being treated with other drugs. This study will examine whether ddI will interact with 566C80 when given in combination.